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DEPARTMENT OF COMMERCE 



Technologic Papers 



OF THE 



Bureau of Standards 

S. W. STRATTON, DIRECTOR 



No. 169 

MEASUREMENT OF PLASTICITY OF 
MORTARS AND PLASTERS 



BY 

WARREN E. EMLEY, Chemist 
Bureau of Standards 



JUNE 28, 1920 




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Monograph 



DEPARTMENT OF COMMERCE 



Technologic Papers 



OF THE 



Bureau of Standards 

S. W. STRATTON, DIRECTOR 



No. 169 

MEASUREMENT OF PLASTICITY OF 
MORTARS AND PLASTERS 



BY 

WARREN E. EMLEY, Chemist 

Bureau of Standards 



JUNE 28, 1920 




PRICE, 10 CENTS 

Sold only by the Superintendent of Documents, Government Printing Office 
Washington, D. C. 

WASHINGTON 

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1920 

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MEASUREMENT OF PLASTICITY OF MORTARS AND 

PLASTERS 



By Warren E. Emley 



CONTENTS 

I. Introduction 3 

II. Definition of plasticity 5 

III. History of previous work 7 

1. Measurement of colloidal content 8 

2. Measurement of viscosity 9 

3. Compressive method 12 

4. Range of plasticity 13 

5. Rate of drying 14 

6. Carson blotter test 15 

IV. Present instrument for measuring plasticity 16 

1. Operation 17 

2. Method of recording results 19 

3. Interpretation of results .....' 19 

4. Numerical expression of results 20 

5. Adopted method of expressing results 22 

V. Plasticities of miscellaneous materials 23 

VI. Effect of consistency upon plasticity 23 

VII. Effect of proportion of sand upon plasticity 25 

VIII. Conclusion 27 

I. INTRODUCTION 

Everyone knows that plasters, mortars, clays, and certain other 
classes of materials are plastic. They must be plastic or they 
can not be used for the purposes for which they are intended. 
On the other hand, few people realize the enormous influence 
which the degree of plasticity has on the economic use of the 
material. 

Take the case of wall plasters, for example. About 70 per 
cent of the total cost of plastering your house is accounted for 
by the labor required to spread the plaster on the wall. If one 
plaster is more plastic than another, it means that the plasterer 
can cover more square yards in a given time with the former 
than with the latter, which, of course, will reduce the cost. Fur- 
thermore, the more plastic material entails less physical and men- 
tal fatigue on the part of the plasterer, and he is thereby led 
unwittingly to produce a better quality of work. 

3 



4 Technologic Papers of the Bureau of Standards 

The material used for the final coat of wall plaster must have 
plasticity in a very high degree. In fact, the requirements are 
so severe that neither Portland cement nor calcined gypsum can 
be used by itself for this purpose; a certain amount of lime must 
be added to give plasticity. This means that dealers must carry 
in stock and deliver on the job two materials, and the plasterer 
must mix these two materials, whereas if either cement or gypsum 
had the desired plasticity, one material would be sufficient. 

While lime in general is more plastic than cement or gypsum, 
different limes have different degrees of plasticity. Any kind of 
quicklime, when properly slaked, will produce a putty of very 
great plasticity. Unfortunately this material is difficult to handle 
by inexperienced operators, so that the phrase "when properly 
slaked " is of undue importance. On the larger operations, espe- 
cially in cities, hydrated lime is rapidly replacing quicklime as 
a plastering material, because of its greater convenience and the 
fact that it is already slaked. Hydrated limes are divided very 
sharply into two classes, on the basis of their plasticity — the 
"finishing" limes, made from a kind of limestone peculiar to a 
small district in northwestern Ohio, and all other hydrates. As 
the art of making hydrate is improved, it is probable that finish- 
ing hydrates will be made elsewhere. In fact, this result has 
already been achieved in one or two cases, to be noted later. 
The plasterer was quick to discover the difference between the 
two kinds of hydrate and to take advantage of it. The result is 
that either putty made from quicklime, or Ohio finishing hydrate, 
is always specified to be mixed with the gypsum for the final coat 
of plaster, with the tendency strongly in favor of the hydrate. 

In 191 5 Ohio finish was being sold in Los Angeles at $26 per 
ton, as compared with $14 for the locally made hydrate. The 
owner who is building a home in Los Angeles is fully cognizant of 
the meaning of plasticity as expressed in dollars and cents. 

Many investigators have attempted to develop finishing hydrates 
outside of Ohio. Several attempts have been made to improve 
the plasticity of Portland cement and of calcined gypsum. These 
investigators have always been handicapped by the lack of 
reliable means for measuring plasticity. It was necessary to 
make experiments on a large scale, so that enough material could 
be produced to permit a plasterer to spread it on a wall. The 
opinion of the plasterer was then recorded as indicating the 
plasticity of the material. The opinion of any man as to the 
amount of work which he performed to accomplish a given result 



mm 



Measurement of Plasticity 5 

is quite undependable. It will vary Math his state of health and 
with his mental attitude, and the opinions of two individuals will 
seldom be found to agree. This method of procedure is therefore 
both unreliable and extremely expensive, so that most investi- 
gators have been compelled to abandon their researches without 
definite results. 

Plasticity of a material is a property for which the consumer 
pays extra. The improvement of the plasticity of materials now 
on the market can readily lead to a saving of money to the con- 
sumer. For these reasons, the measurement of plasticity is not 
a question of academic interest only, but is of real practical 
importance to everyone who uses or pays for any mortar or 
plaster. 

II. DEFINITION OF PLASTICITY 

It seems strange that a property so important and so generally 
well known as plasticity should be so difficult to define that it has 
become the subject of endless discussion. Everyone who has made 
no special study of the subject can tell whether or not a material is 
plastic. But when one does make a special study of plasticity, 
the definition which he evolves is apt to be colored by the appli- 
cation of the material with which he is dealing, or by the view- 
point from which his research was undertaken. 

A broad general definition, states that "plasticity is that 
property of a material, or combination of materials, by virtue of 
which it deforms continuously and permanently during the 
application of force. " * 

Unfortunately, this is not sufficiently detailed for everyday 
use. Its greatest fault lies in the fact that plasticity so defined 
is not comparable. According to this definition, a material either 
is plastic or it is not; it can not be more or less plastic. 

Engineers, who approach the subject from the viewpoint of 
a solid, emphasize the fact that the deformation of a plastic 
material must be permanent, in contradistinction to the deforma- 
tion of an elastic material, which is not permanent. 2 

Other investigators, starting from the viewpoint of a viscous 
liquid, find that a plastic material vvill sustain a slight initial 
load without deformation, while a true liquid will not. 3 

The users of plastic materials have their own ideas on the 
subject, and these ideas differ with different trades. Thus, a 

1 Report of Committee C-7, Amer. Soe. Test. Mat.; 1919. 

2 Johnson, Materials of Construction, p. 1; 1915. 

• Bingham and Green, Paint, a Plastic Material and not a Viscous Liquid, Proc. Am. Soc. Test. Mat.; 
1919. 



6 Technologic Papers of the Bureau of Standards 

clay worker may speak of a clay as being too plastic, whereas 
such a term would appear absurd to a plasterer. 

To arrive at a definition which can be used to establish a basis 
of measurement, it seems necessary, therefore, to confine ourselves 
to the particular uses, as plasters and as mortars, of the plastic ma- 
terials . If such a definition enables us to evolve a machine which will 
correctly measure plasticity, then it is highly probable that the same 
definition and machine can be used for materials other than 
plasters and mortars, even though such use may involve a read- 
justment of generally accepted nomenclature. If a machine 
which is designed to measure the plasticity of plasters will not 
also measure the plasticity of sands or clays, with only minor 
changes to enlarge its scale, then there is something fundamentally 
wrong with the principle on which the machine was built and with 
the definition which dictated that principle. 

From the viewpoint of a plasterer or mason, no dry material 
is plastic; it must be mixed with water to develop its plasticity. 
Mortar or plaster is usually applied to a surface which is more 
or less absorbent and which sucks the water out of it. The 
material loses plasticity at the same rate that it loses water. 
Therefore, the plasticity of a material depends directly upon its 
ability to hold its water against the suction of the surface to 
which it is applied. 

Visualize a trowel full of plaster applied to an absorbent wall. 
The layer of the plaster in direct contact with the wall will lose 
its water (and its plasticity) immediately. Each successive 
layer, counting outward from the wall, will be able to hold its 
water a little longer than the one under it, because the water in 
each layer must percolate through an increasing thickness of 
material in order to reach the wall. That is, the time required 
for the wall to suck the water out of all the plaster will depend 
upon the thickness of the pat of plaster. When the water has 
been reduced to a certain proportion, the plaster is no longer 
workable. Some plasters can be spread out much more thinly 
than others. The plasterer measures this property quite accu- 
rately by noting the number of square yards of surface that can 
be covered by a given volume of plaster. The more plastic the 
material the greater the yardage. 

The rapidity with which plaster of given thickness will lose 
its water will depend upon the ability of the drier layers next 
to the wall to obstruct the passage through them of the water 



Measurement of Plasticity 7 

coming from the outer layers; that is, upon the inherent ability 
of the material to retain its water. Thus the yardage is a meas- 
ure of that factor of plasticity which was noted in the preceding 
paragraph. This factor has sometimes been considered as all 
important, and plasticity has been defined as being the ability 
to retain water against suction. 4 

Another factor which must be considered is the amount of 
work required to spread the plaster. Some plasters are noto- 
riously sticky, while some work freely and smoothly under the 
trowel. This factor has sometimes been considered the most 
important, resulting in the definition that "that material is the 
most plastic which can be spread with the least work." 5 

Our definition, therefore, must contain two parts, as follows: 
(i) That material is the more plastic which has the greater 
ability to retain its water against the suction of the surface to 
which it is applied; (2) that material is the more plastic which 
requires the less work to spread it. A correlation of these two 
factors will be attempted later. 

Plastic materials may be distinguished among themselves 
according to their working qualities. This leads to a new and 
more limited definition of plasticity, in that a plastic material 
is one which works freely and easily under the trowel and has 
marked ability to hold its water. In distinction to this, there 
are sticky materials which hold their water, but pull and work 
"rubbery" under the trowel, and sandy materials which work 
harshly and dry out quickly. This gradation of plasticity — 
sticky, plastic, sandy — is the one generally accepted in the 
trade. 

III. HISTORY OF PREVIOUS WORK 

Work on the development of a means of measuring plasticity 
was started by this Bureau in 1909, and has been continued 
more or less steadily ever since. Altogether some 20 different in- 
struments have been designed, built, experimented with, and 
eventually scrapped, each instrument representing one more step 
in our knowledge of the subject. 

In the present state of general knowledge it is not difficult to 
build a machine and say that it will measure plasticity. The 
difficulty lies in offering convincing and acceptable proof that the 

* Emley, Practical Method (or Comparing the Working Qualities of Hydrates, Trans. Nat. Lime Mfr::. 
Assn.; 1915. 
6 Emley. Measurement of Plasticity, Trans. Nat. Lime Mfrs. Assn.; 1917. 



8 Technologic Papers of the Bureau of Standards 

machine will fulfill the claims made for it. For this reason it is 
thought advisable to go into the past history of the work, leading 
the reader up through successive failures, pointing out the sources 
of error in each case, and finally establishing in his mind the con- 
viction that the present instrument will really measure plasticity. 
The methods tried have been based upon seven fundamentally 
different principles: (i) Measurement of colloidal content; (2) 
measurement of viscosity; (3) compressive method; (4) range of 
plasticity; (5) rate of drying; (6) Carson blotter test; (7) present 
instrument for measuring plasticity. 

1, MEASUREMENT OF COLLOIDAL CONTENT 

Some years ago, when the chemistry of colloids first attracted 
attention, the theory was evolved that the plasticity of a material 
is directly dependent upon the proportion and kind of colloidal 
substance which it contains. Extended and successful efforts 
were made to measure the colloidal content of clays. 6 The 
methods employed consisted of deflocculating the colloidal matter 
by the addition of minute quantities of acid or alkali, or by 
measuring the quantity of organic dye which the colloidal matter 
could absorb. 

Under Mr. Ashley's personal supervision, these same methods 
were applied to lime, but without success. The lime itself is so 
strongly alkaline in character that it was found impossible to 
deflocculate it with any reagent. The dye found most satisfactory 
for clays — malachite green — is an oxalate. It entered into chem- 
ical reaction with the lime and was immediately and completely 
decolorized. Some eighty other dyes were tried, but none was 
found which gave any assurance that the quantity of color removed 
by the lime was due to absorption and not to chemical combination. 

If we concede that colloidal matter is merely matter in an 
extremely fine state of division, and agree that the finer the size 
of the individual particles the greater will be the area of their 
surfaces per unit of volume, then the measurement of the so-called 
"surface factor" becomes in effect a measure of the colloidal 
content. This surface factor theory is that the plasticity of a 
material is dependent upon the area of the surfaces of the grains. 
The sizes of the grains are determined as far as possible by sieves, 
and the finest particles are graded by elutriation. 

6 Ashley, H. E., The Technical Control of the Colloidal Matter of Clays, B. S. Technologic Paper No. 23; 
1912. 



Measurement of Plasticity 9 

Unfortunately, this method is also inapplicable to hydrates. 
The grains are mostly so small that even the finest sieve is not of 
much use in separating them. Elutriation, which depends upon 
the carrying capacity of streams of water of different velocities, 
did not give definite results because of the solubility of the lime 
in the water. 

It is well known that the plasticity of a lime changes quite 
rapidly when the lime is soaked with water. If the colloidal 
theory is correct, then this change in plasticity should be accom- 
panied by a change in the size of the grains. It was therefore 
decided that there are too many difficulties in the way to permit 
measuring the sizes of these very small grains when the sizes are 
changing continuously and rapidly. 

As a summary of our experiments with this method, it may be 
stated that the plasticity of lime may be, and probably is, depen- 
dent upon the sizes of grains, but the experimental difficulties in 
measuring these sizes have to date been insurmountable. 

2. MEASUREMENT OF VISCOSITY 

It is generally accepted that a plastic material is composed of 
inert solid particles suspended in a liquid medium. The viscosity 
of the liquid should determine the freedom with which the solid 
particles can move, and should therefore influence the plasticity 
of the mass. 

On this assumption it was decided to investigate the viscosities 
of lime pastes. For this purpose it was first attempted to use a 
modification of the apparatus used by Arndt for measuring 
viscosities of melted silicates. 7 The instrument is illustrated by 
the accompanying drawing (Fig. 1 ) . A plunger of known dimensions 
is immersed in the liquid, and then pulled out by the falling of a 
known weight. The viscosity is proportional to the time required 
to pull the plunger out of the liquid. 8 The instrument was found 
to be applicable only through a very limited range of viscosities. 
When the weight operating the plunger was set for thick pastes, 
it pulled the plunger out of thin pastes so rapidly that the time 
could not be measured with sufficient accuracy. The dimensions 
of the instrument were purely empirical; it was impossible to 
translate the results into absolute units. Hence, when the plunger 

7 Doelter and Sirk, Determination of Absolute Values of Viscosities of Melted Silicates, Mouatshefte 
fur Chemie, pp. 32 and 643; 1911. 

8 Emley, Tests of Commercial Limes, Trans. Nat. t,ime Itfrs. Assn.; 191,5. 

181119°— 20 2 



IO 



Technologic Papers of the Bureau of Standards 



or the weight was changed, the results immediately became 
incomparable. 

While the plunger viscosimeter was in use, a new instrument, 
based on an entirely different principle, was being designed. 
This instrument rotated a plunger or paddle immersed in the 
liquid, and measured the force required to turn it at a constant 



~rr 



331 



5=n 



larh 



I 



5* 



Fig. i. — Plunger viscosimeter 

speed. Its final design is indicated by the accompanying draw- 
ing (Fig. 2). 9 Great credit is due to Mr. Clark, the designer of 
this instrument, for the ingenuity with which he was able to 
overcome all of the difficulties previously encountered. In 
order to reduce friction to an absolute minimum, the paddle 
was mounted on jewel bearings. The necessity of mechanism to 



•Emley, The Clark Viscosimeter, Trans. Ainer. Ceram. Soc; 1913. 



Measurement of Plasticity 



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turn the paddle was eliminated by using a magnetic drive. Even 
the friction of a speed indicator was done away with, the speed 
being measured by optical illusion. This instrument provided 
means whereby the viscosities of lime pastes could be measured 
throughout their entire range from thin to thick. The results 
could be compared to the viscosity of water as a standard, or 
could be calculated in absolute _ 

cgs units. 

Both the plunger and the 
Clark viscosimeters were used 
for some time, making slight 
changes in the design. Five 
different shapes of plungers 
were tried on the former. The 
Clark instrument was entirely 
rebuilt once, and was provided 
with two shapes of paddles. 10 

From these numerous experi- 
ments, it was finally concluded 
that, while the viscosity of the 
liquid medium may influence 
the plasticity of the mass, it is 
not the governing factor. It 
is quite possible, by properly 
proportioning the lime and 
water, to produce, from any 
two limes, pastes having the 
same viscosity. Any plasterer 
will confirm the statement 
that the plasticity is depend- 
ent upon the quality of the 
lime, rather than upon the 
quantity of water. 

Of course, some exception may be taken to the conclusions 
reached, because the viscosity was not measured in the usual 
manner — the rate of flow through a capillary tube. Obviously 
it is impossible to force lime pastes, which contain large particles, 
to flow through capillary tubes. They can be made to flow 
through tubes of measurable dimensions, and this method has 



Fig. 2. — Clark viscosimeter 



10 Emlcy, Effect of Consistency and Amount of Sand on the Properties of Liine Mortars, Trans. Amer. 
Ceram. Soc. ; 1914. 

11 Bleininger and Ross, Flow of Clays Under Pressure, Trans. Amer. Ceram. Soc.; 1914. 



12 Technologic Papers of the Bureau of Standards 

been used on clays 11 and limes. 12 However, even by this method, 
no definite relation has been shown to exist between viscosity 
and plasticity. 

More recent work on this line indicates that, a plastic solid 
differs from a viscous liquid in that the solid is able to sustain a 
slight initial force before it begins to deform, but after deformation 
has begun, the solid behaves in the same manner as the liquid — 
the rate of flow through a capillary tube is directly proportional 
to the force causing the flow. 13 This brings in another factor 
which had previously been overlooked. Plastic bodies may 
differ from each other not only in their viscosities, but in the 
magnitudes of the initial forces required to start deformation. 
While the apparatus used by Bingham and Green is inapplicable 
to limes because it involves flow through a capillary, it is noted 
that Bleininger and Ross observed this same phenomenon, the 
existence of a "yield value." Probably measurements made by 
the method of Bleininger and Ross and interpreted according to 
the formula of Bingham and Green would supply the missing 
link which is necessary to connect the viscosity of lime paste 
with its plasticity. 

It seems inevitably true that the viscosity of a lime paste will 
depend chiefly on the ratio of lime to water. When this ratio is 
held within such limits that the paste is workable under the 
trowel, we know from experience that plasticity is not dependent 
upon this ratio, to any great extent. Therefore the measure- 
ment of viscosity, while it would indicate the magnitude of one 
factor entering into plasticity, could certainly not be expected to 
tell the whole story. For this reason the measurement of viscosity 
was finally abandoned. 

3. COMPRESSIVE METHOD 

Our next attack on the subject was started from the view- 
point of the engineer, dealing with plastic solids. In Merriman's 
text book on the Mechanics of Materials, page 378, the statement 
is made that when a plastic body is subjected to increasing com- 
pressive loads it will first deform and eventually rupture. The 
rupture will occur along a certain well-defined plane. The angle 
which this plane makes with the vertical is a characteristic of 
the material, and, when considered together with the load 

13 Lazell, E., Private communication. 

13 Bingham and Green, Paint, a Plastic Material and not a Viscous Liquid, Proc. Am. Soc. Test. Mat.: 



Measurement of Plasticity 13 

required to cause rupture, affords a definite and reliable means 
for distinguishing between plastic bodies. The theory has been 
worked out mathematically to produce two formulas: 

N = cot 26 
S = S tan 8 

2N 

8 is the angle which the plane of rupture makes with the vertical, 
and 5 is the load per unit of area required to cause rupture. 
These formulas enable us to calculate two inherent properties of 
a plastic material: N is the "coefficient of internal friction," 
and S is the "unit cohesive strength." Translating these terms 
into ordinary usage, when N is high, the material is sticky; when 
N is low, the material is sandy; when S is low, the material is 
short working. The conditions for maximum plasticity are an 
intermediate value for N and the highest possible value of S . 

An instrument was built to measure the compressive strength 
of green lime pastes and mortars. It is shown in the photograph, 
Fig. 3. It operates on the same principle as an Olsen testing 
machine on a very small scale. 

A great many experiments were made with this instrument, the 
results being published in three papers. 14 It was found that the 
method does give valuable information about the plasticity of lime, 
but, like the measurement of viscosity, it does not tell the whole 
story. The question of consistency again looms up like an 
insurmountable obstacle. With a very thick paste, 5 is large 
and 8 is nearly zero. As the paste is thinned down by the addi- 
tion of water, S decreases and 8 increases, until, when the body 
changes from a plastic solid to a viscous liquid, S becomes equal 
to zero and 8 to 45 °. In other words, the plasticity of a mate- 
rial, when measured by this method, depends upon the ratio of 
lime to water, which is not true. The method was therefore 
abandoned. 

4. RANGE OF PLASTICITY 

The experiments with the compressive method emphasized 
the previously known fact that plastic materials vary in the 
length of their plastic range. If increasing amounts of water 
are added to hydrated lime, the changes in consistency will 
occur in the following order: dry powder, damp powder, sticky 

" Emley, Deformation of Plastic Bodies Under Compression as a Measure of Plasticity. Trans. Amer. 
Ceram. Soc, 1915; Measurement of Plasticity of Hydrated Lime by the Compression Method, Tr.ms. Nat. 
Lime Mfrs. Assn., 1915; Compressive Method of Measuring Plasticity, Trans. Nat. Lime Mfrs. Assn., 1916. 



14 Technologic Papers of the Bureau of Standards 

mass which when molded is practically a solid, plastic material, 
liquid. Suppose a given hydrate changes from a solid to a plas- 
tic material when mixed in the proportion, 80 per cent lime, 
20 per cent water. When the proportion is 50 per cent lime, 
50 per cent water, the material changes from a plastic material 
to a liquid. Then the " plastic range " of this lime is 80 — 50 = 30 
per cent. It has been found by experience that, as a general 
rule, the more plastic the material the greater will be this range. 
This method has been employed for many years in the study of 
clays, under the title of the "Atterberg Plasticity Method." 15 
The compressive method described above provided quite accu- 
rate means for determining the end points of the plastic range, 
and the results obtained from experiments with the compressive 
method could be used as data for the comparison of plastic ranges. 
From a study of these data the following conclusions were 
drawn: (1) Limes as a class follow the general rule, the more 
plastic the lime the greater the range. (2) The end-points can 
not be determined with sufficient accuracy to permit differentia- 
tion between similar limes. Differences which are quite notice- 
able under the trowel can hardly be detected by this method. 
(3) The method is entirely inapplicable and misleading when it is 
used as a basis of comparison for dissimilar substances, such, for 
instance, as neat lime and lime mortar. This last conclusion, 
which really eliminates further consideration of the method, has 
been verified by other investigators. 18 

5. RATE OF DRYING 

The quantity of water which must be mixed with a lime to 
render it plastic is conceivably a function of the size of grain of 
the lime and is to that extent a measure of its colloidal content. 
If plasticity depends upon the presence of colloidal matter, then 
the quantity of water required should be at least in some degree a 
measure of plasticity. This would probably be true if it were not 
for two factors: (1) The quantity of water is not definite for any 
given material; there is a range in the proportions of lime and 
water within which all mixtures are plastic. (2) Plasticity is not 
only quantitative, but also qualitative in its nature. It is im- 
possible, by the mere addition of water, to bring all limes to the 
same degree of plasticity. 

15 Albert Atterberg, International Reports on Pedology; 1911. 

16 C. S. Kinnison, Study of the Atterberg Plasticity Method, Trans. Amer. Ceram. Soc.; 1914. 



Measurement of Plasticity 15 

Plasters are used by spreading them on absorbent surfaces. 
They must be mixed with enough water to make them plastic, and 
they must be able to retain that water against the suction of the 
surfaces on which they are spread. The length of time during 
which a plaster can hold enough water to maintain its plasticity 
will depend not only on the quantity of water originally present, 
but also on some inherent property of the plaster which gives it 
the ability to retain water. 

An attempt was made to measure this property of hydrates. 
Pats of lime paste were spread on plaster blocks. The blocks 
sucked the water out of the lime. The rate of drying was meas- 
ured by sticking a needle into the paste every few seconds. The 
method was found to give very promising results. 17 It was refined 
to some extent by molding the lime paste in the form of a wedge, 
so that the rate of drying could be followed up the wedge, and the 
thickness of the dried paste could be plotted against the time. 18 

From the reasoning given above, it will be understood that this 
method was not intended to measure plasticity. It measures the 
ability of the material to retain its water, which is only one of the 
factors governing plasticity. The method performed very much 
better than was expected; it differentiated sharply between Ohio 
finishing hydrates and nonplastic hydrates. This is taken to mean 
that the rate of drying is a measure of the most important factor 
governing plasticity. So predominant is the influence of this fac- 
tor that the method forms a reliable basis for the comparison of 
different classes of materials 

The ability to retain water, while the most important, is not the 
only factor governing plasticity. The measure of this ability 
permits distinction between classes of materials, but the other 
factors must be considered when attempting to compare materials 
in the same class. 

6. CARSON BLOTTER TEST 

During an informal discussion at the 1916 meeting of the 
National Lime Manufacturers' Association, Mr. W. E. Carson, 
who was then president, described a test which he had found satis- 
factory for the measurement of plasticity. This is essentially a 
duplication on a small scale of the action of a plasterer when 
spreading plaster on a wall. The wall is represented by a sheet 

17 Emley, Practical Method for Comparing the Working Qualities of Hydrates, Tr.ms. Nat. Lime Mfrs. 
Assn.; 1915. 
18 Kirkpatrick, F. A., Circular Letter to Members of Committee C-7. Am. Soc, Test. Mat.; 1917. 



1 6 Technologic Papers of the Bureau of Standards 

of blotting paper; the trowel by a case knife or a spatula. This 
method was immediately investigated, pronounced satisfactory, 
and is at present in general use. 

The simplicity of the operation and of the apparatus are great 
points in its favor. Anyone anywhere can procure a piece of 
blotting paper, a tin cup, and a table knife. Mix the lime with 
water in the cup. Use enough water to make a good plastering 
consistency, although this point is not very important. It is 
usual to let the paste soak overnight before testing it, in accord- 
ance with the custom of soaking hydrate overnight before using 
it. In the morning try to spread the lime out on the blotter with 
the knife and note " how it works." 

In the hands of an experienced operator this method is per- 
fectly trustworthy. Very small differences in plasticity can be 
detected. 

While this method is excellent for regular routine work, such as 
checking up the quality of a day's output by the hydrate-mill 
foreman, or the comparison of two brands of hydrate, its use as 
an instrument for research is seriously handicapped. It is prac- 
tically impossible to express the results in such a way that they 
can be understood by anyone else. This means that each operator 
must be taught personally by someone with experience, else he 
will have difficulty in recognizing the distinguishing character- 
istics. Having learned how to make the test, the operator must 
have considerable practice to gain the necessary assurance that 
his work is correct. The difficulty of expression makes it im- 
practicable to record the results. It is impossible to test one 
lime and compare its behavior with that of another lime which 
had been tested a week before or by another operator. Compari- 
sons can be made only when both samples are tested at the same 
time by the same operator. 

IV. PRESENT INSTRUMENT FOR MEASURING PLASTICITY 

The Carson blotter test enables one to measure the rate of dry- 
ing of the lime, and also the work required to spread the lime on 
the wall before it has dried. These are the two factors of plas* 
ticity included in our definition and recognized by plasterers and 
masons. The former is much more important than the latter (as 
shown by the results of the wedge test), but the latter must not 
be overlooked. 



^Measurement of Plasticity 17 

In order to measure both of these factors at the same time, it 
was decided to build a machine which would duplicate the action 
of the plasterer. By designing this machine in such a way that 
the force exerted by the plasterer can be measured, it becomes in 
effect a blotter test refined so that the personal equation is elimi- 
nated and the results are recordable. 

Such an instrument was built in 191 6. A photograph of it is 
shown in Fig. 4. A few experiments with it demonstrated some 
mechanical imperfections, so that it was redesigned. The second 
design, and the principle upon which the instrument operates, were 
described in two papers, 19 but the second edition of the instru- 
ment was not completed and ready for operation until August, 
1919. Its construction is illustrated by the photograph Fig. 5. 

1. OPERATION 

The operation of the instrument may be briefly described as 
follows : A number of interchangeable plaster blocks are provided 
to be used as base plates. These furnish the absorbent surface, 
corresponding to the wall in practice and to the blotter in the 
Carson blotter test. They are carefully made so that they all 
have about the same absorption, and are completely dried in an 
oven at 70 C after each experiment. The sample is mixed with 
enough water to make a good workable paste. The consistency 
of the mix is of some, though minor importance. This feature 
will be discussed later, under Results of Experiments. The paste 
is molded in the form of a cylinder 3 inches in diameter by \% 
inches high, using one of the plaster blocks as a base plate. The 
mold is immediately removed, and the plaster block carrying its 
cylinder of paste is placed in position on the permanent platform 
of the instrument. The whole is then moved upward by hand 
until the top of the paste just comes into contact with the bottom 
of the disk suspended above it, when the experiment is ready to 
begin. This operation must not occupy more than \ l / 2 minutes 
from the time the first batch of sample is put into the mold to the 
time when the experiment is begun by throwing the switch to 
start .the motor. The permanent platform which carries the 
sample is mounted on the upper end of a vertical screw. The 
motor turns this screw, which travels through a fixed nut, so 
that it moves upward as it revolves. The speed is such that the 

19 Emlcy, An Instrument for Measuring Plasticity, Tran. Amer. Ceram. Soc, wir; An Instrument 
for Measuring Plasticity, Trans. Nat. Lime Mfrs. Assn., 1917. 



1 8 Technologic Papers of the Bureau of Standards 

specimen makes one revolution in 6}4 minutes, and moves up- 
ward one-thirteenth of an inch in the same length of time. The 
upward motion presses the surface of the specimen against the 
disk. This disk is conical in form, mounted point downward on 
a vertical shaft. It is free to turn, but can not move upward. 
This conical disk is supposed to represent the trowel. Its sides 
make an angle of io° with the horizontal, this being assumed to 
be about the angle that a plasterer's trowel makes with the wall. 
While the disk is free to turn, each increment of rotary motion 
requires an increasingly larger force to accomplish it. A cord 
connects the shaft of the disk to a pulley, to which a pendulum 
arm is rigidly attached. As the disk turns it winds up the cord, 
pulls the pendulum more and more away from the vertical and 
toward the horizontal, and thereby continuously increases the 
force required to produce further motion. The pendulum carries 
a pointer which moves over a circular scale, so that at any time 
the angle which it makes with the vertical ccn be read. The 
force which is being exerted to turn the disk is directly proportional 
to the sine of this angle. The force is also directly proportional 
to the weight of the pendulum; the magnitude of the scale readings 
can be changed by using bobs of different weights attached to the 
pendulum. Two bobs are provided, weighing 350 and 800 grams, 
respectively. 

The upwaid spiral motion of the specimen against the disk 
results in a twisting moment, which tends to spread the specimen 
out on the plaster block. The pendulum provides means of meas- 
uring that part of the force exerted which is acting parallel to 
the surface of the specimen. The absorption of the plaster 
block gradually removes water from the specimen and permits 
measurement of the rate of drying. 

From the drawing and photograph, it will be noted that three 
changes have been made in the instrument since it was built: 
(1) It was originally intended to permit the disk to move upward, 
and to measure the force required to cause motion in this direc- 
tion. A large number of experiments with the first edition of 
the instrument demonstrated conclusively that the magnitude 
of this force is not interesting. This mechanism has therefore 
been locked so that it will not work, and it may well be omitted 
from further designs. (2) The design calls for a flat disk as well 
as a conical one. A large number of experiments were run with 
this flat disk, but it was found that the conical one is more satis- 




.8 

Si 







s 



-O, 



a 



o 



Measurement of Plasticity 19 

factory for plasters and mortars, though the former may possibly 
give some valuable information about clays. (3) The original 
design calls for a speed of 3 rpm. This was found to be much too 
fast for plasters, and the speed now in use — one revolution in d% 
minutes — has been finally adopted as satisfactory. It may be 
found necessary to increase this speed when dealing with ma- 
terials which set very quickly (unretarded calcined gypsum, for 
instance), but determinations made at different speeds are not 
comparable. 

2. METHOD OF RECORDING RESULTS 

While the instrument is in operation, the specimen is continu- 
ously drying out. This, together with the continuously increasing 
area of contact between the specimen and the conical surface of 
the disk, results in a continual increase in the force tending to 
turn the disk. The pendulum gradually swings out over its 
circular scale. The results of an experiment are expressed as 
the relation between time and force. The time is counted in 
minutes, beginning with the time when the first lot of the specimen 
was put into the mold. Since one bob only is used during any 
experiment, the force is recorded as the sine of the angle which 
the pendulum makes with the vertical. It is convenient to express 
the results in the form of a curve, using time as the abscissa and 
force as the ordinate. A large number of experiments, conducted 
during a period of three years, have enabled us to recognize the 
typical curves of plastic and nonplastic hydrates. The former will 
start to rise late and will continue to rise gradually. The latter 
will start to rise early and will continue to rise abruptly. The 
shape and position of this curve constitute criteria of plasticity 
which all of our experiments have shown to be infallible. 

3. INTERPRETATION OF RESULTS 

If, for example, one is conducting experiments to improve 
the plasticity of a material, each change in the manufacturing 
process can be followed up by the instrument and the curve 
obtained can be filed away for future reference. This ability to 
refer back to previous results is of great practical importance. 
Herein lies the great advantage of the instrument over the Carson 
blotter test. 

The use of a curve to express results, has, however, many 
disadvantages. It requires some little study of a curve to deduce 
its exact meaning, and its use is therefore inconvenient. While 



2o Technologic Papers of the Bureau of Standards 

the comparison of two curves usually enables us to tell without 
doubt which lime is the more plastic, the relative degrees of 
plasticity are still indeterminate. An attempt has been made to 
draw an arbitrary line across the curve sheet and state that when 
a curve lies to the right and below this line the hydrate can be used 
as a finishing lime; when a curve lies to the left and above the 
line, the hydrate is nonplastic. This attempt has thus far been 
entirely successful, but there is always the possibility of a quibble 
about the position of the arbitrary line. If one curve crosses 
another in two places, as frequently happens with similar materials, 
it is impossible to tell from the curves which of the two substances 
is the more plastic. 

For these reasons, many attempts have been made to find 
numerical expressions for the curves. It must be emphasized 
that these attempts are merely for the sake of greater convenience, 
and that the curves remain as the final criteria. 

4. NUMERICAL EXPRESSION OF RESULTS 

It will be remembered that the speed of the machine is constant. 
The abscissa representing time can, therefore, by a suitable change 
in the scale, be made to represent distance. The area under the 
curve then becomes the product of the force and the distance 
through which it acts, which, by definition, is the work done. 
If we accept the old statement that that lime is the most plastic 
which can be spread on the wall with the least work, then the 
area under the curve is a direct measure of plasticity — the less 
the area the more plastic the material. 

Unfortunately, this area could not be directly measured. 
There was no right-hand end to the curve, no stopping place. 
The force continued to increase with the time indefinitely. An 
attempt was made to remedy this defect by setting an arbitrary 
time limit. The work done during the first five minutes was 
established as a basis for comparison. 20 The results were ex- 
tremely gratifying, so much so that this method was on the point 
of being adopted as standard. Further study of it, however, 
brought to light two serious fallacies: (i) In a majority of cases 
the curve could not be continued for five minutes. Either the 
specimen ruptured or else the force ran off the scale. It was 
necessary, therefore, to extrapolate the curve in order to measure 
its area for five minutes. This is of doubtful expedience in any 

20 Kirkpatrick and Orange, Tests of Clays and Limes by the Bureau of Standards Plasticimeter, 
Jour. Amer. Ceram. Soc., March, 1918. 



Measurement of Plasticity 21 

case, and in cases where the specimen ruptured it is positively 
wrong. (2) It requires very little work to spread pure sand, if 
the operation can be completed before the sand has dried out. 
This method would therefore show sand to be one of the most 
plastic materials, which of course is absurd. The trouble here 
is not with the method, but with the definition. That material 
which can be spread with the least work is not necessarily the 
most plastic. The thinness to which the material can be spread, 
the time during which it can be worked, its ability to retain water 
(all of which terms are synonymous), is a more important factor 
in plasticity than is the quantity of work done on the material. 
While it requires less work to spread sand than lime, the sand 
dries out so quickly that it can not be spread to the same thin- 
ness nor worked for the same length of time and is therefore less 
plastic. 

It is absolutely necessary, therefore, to find a right-hand end 
to the curve, not an arbitrary one, but one which depends upon 
the inherent character of the material. This has been accom- 
plished with the second edition of the instrument. By using 
a slower speed and a heavier bob, it has been found possible to 
reach the point of rupture of every specimen thus far examined. 
The force does not continue to increase with the time indefinitely. 
A point is eventually reached where, due to the largeness of the 
force and the dryness of the material, the specimen breaks. This 
break is plainly visible, and is indicated in the results as the 
point where the curve abruptly ceases to rise. 

Having now a definite end point to the curve, it is a simple 
matter to compare the quantities of work which these, curves 
represent. Here again the argument leads to absurdity. The 
work done on sand is very much less than the work done on lime, 
yet the lime is more plastic. 

Reverting back to the preceding discussion, it will be remem- 
bered that plasticity is dependent upon two factors, the time dur- 
ing which the material can be worked and the work required to 
spread it during that time. Of the two the former is the more 
important. By considering only the area of the curve we have 
neglected the first and more important factor. Mathematically 
it can be stated that plasticity varies directly as the time and 
inversely as the work. The work, however, is represented by the 
area, force multiplied by time. A combination of these two state- 

ments would indicate that P=Kr== = -=; y or the plasticity varies 



22 



Technologic Papers of the Bureau of Standards 



inversely as the force and the time element is completely elimi- 
nated. This, again, is absurd. The trouble is that our mathe- 
matics have given equal values to time and work, whereas we 
know that the former is more important and should carry more 
weight than the latter. The T in the denominator should not be 
able to cancel completely the T in the numerator. 

5. ADOPTED METHOD OF EXPRESSING RESULTS 

We have not been able to determine the relative importance of 
time and work. We have been able to establish the fact that the 
former is of far greater importance than the latter — is, in fact, 
the predominating factor. If we consider two plasters, one of 
which can be worked on the wall for 30 minutes and the other for 



4> 600- 



°Z500' 



'4O0- 



'300- 



si 

'5 
5 

q/00- 




35 40 4S 

Time: minutes. 



/. - Cat. after soaking. 

2. - Finishing hydrat^ aftersoaking 

3- Cat. before soaking. 

4. - pehydrated day- 
S-Finishing hydrate. be fore soaking 
G.-Masonshydrafe, aftersoaking. 
7-Cellle. ' 

8. -Masonshydrate. before soaking. 
9- 1 -4 lime mortar. 
10. - Portland cement. 
Il.-Sand. 

TO 



Fig. 6. — Plasticity curves of miscellaneous materials 

only 15 minutes, there is no doubt that the former would be 
generally accepted as the more plastic, regardless of the relative 
quantities of work required to spread the two. If, on the other 
hand, two plasters can be worked for exactly the same length of 
time, then that is the more plastic which requires the less work 
to spread it. 

Acting on this theory, plasters have been divided into classes 
according to the length of time during which they can be worked, 
and the plasters within each class have been ranked according to 
the work required to spread them. 

As a matter of convenience, 5-minute intervals have been 
fixed as a basis of classification. The curves of some miscella- 
neous materials are shown in Fig. 6, and the results are classified 
in the table given below. 



Measurement of Plasticity 23 

V. PLASTICITIES OF MISCELLANEOUS MATERIALS 

The following table calls attention to certain facts. Before 
soaking, the finishing hydrate is not much better than the non- 
plastic. Soaking raises the finishing hydrate three classes, the 
nonplastic hydrate only one class. Materials in the 20-minute 
class or better can be used as finishing plasters, those below the 
20-minute class can not. 

TABLE 1. — Relative Plasticities of Miscellaneous Materials 





Relative 






Relative 






Class 


force 
required 
to spread 


Material 


Class 


force 
required 
to spread 


Material 




30-minute 


14 


"Cal" a, soaked overnight 


10-minute 


12 


Celite 






51 


Finishing hydrate, soaked 
overnight 




13 


Nonplastic hydrate, 
soaked 


not 


25-mlnute 


20 


"Cal" not soaked 




15 


1 : 4 lime mortar 




20-minute 


37 


Dehydrated clay 




16 


Cement No. 1 




15-minute 


24 


Finishing hydrate, not soaked 




16 


Cement No. 2 






34 


Nonplastic hydrate, soaked 
overnight 


S-minute . 


3 

5 


Cement No. 3 
Very fine sand 





a "Cal" is a proprietary material which is intended to be added to concrete to accelerate its early 
hardening. 

Experimental evidence. — " The proof of the pudding is in the 
eating." The principle upon which the plasticimeter operates 
appears to be fundamentally sound, but if the results obtained by 
its use are erroneous, the theory will not be able to hold its own 
against the facts. On the other hand, criticism of the theory 
must be expected ; but, if it is shown that the results obtained are 
correct, this will afford the best possible means of substantiating 
the theory. 

In order to prove that the instrument does give correct results, 
experiments have been conducted along two different lines — 
a determination of the effect of consistency upon the plasticity 
of lime pastes and a determination of the effect of the quantity 
of sand on the plasticity of lime mortars. 

VI. EFFECT OF CONSISTENCY UPON PLASTICITY 

It was noted above that consistency has little influence on 
plasticity. If a lime paste is not plastic, no amount of water 
which may be added to it can render it so. The converse of this 
is also true; a plastic lime remains plastic, regardless of the 
quantity of water which may be added to it. Of course, these 



24 



Technologic Papers of the Bureau of Standards 



statements are true only when the material is really plastic — when 
it does not approach too closely to either the solid or liquid condi- 
tions. Any plasterer will verify these statements, and they can be 
corroborated with a trowel at any time. 

The consistency does, however, have some influence. Obvi- 
ously, the time required to dry the water out of a plaster, while it 
depends chiefly on the ability of the plaster to retain water, must 
be somewhat dependent upon the quantity of water originally 
present. Experience shows that a wet paste will have somewhat, 
though very little, better spreading qualities, than a drier one 
made of the same material. 



criur 

c e . 

<£ (0 so- 

ym Jo- 




is 



20 



22 



& IO IS 14 16 

Time: minutes 

Fig. 7 . — Effect of consistency on plasticity 

To illustrate this effect, and at the same time obtain some idea 
of its magnitude, a series of specimens were tested, all made of the 
same lime, but with different proportions of water. The results 
are shown by the curves, Fig. 7, and are tabulated as follows: 

TABLE 2.— Effect of Consistency Upon Plasticity 



Per cent 
lime 


Per cent 
water 


Class 


Force 


Per cent 
lime 


Per cent 
water 


Class 


Force 


51 


49 
47 
45 


20-minute 


34 
14 
28 


57 
59 


43 
41 




23 


53 




10-minute 


33 


55 















Evidently, the wetter the consistency the more plastic the ma- 
terial. In practice the consistency must be at least dry enough 
so that the plaster will not run off the trowel. The wettest speci- 
men included in the above series is possibly a little wetter than 
could be readily handled. It is impracticable, therefore, to make 
from this hydrate a paste which would have the same plasticity 
as that shown by the finishing lime in Fig. 6. 



Measurement of Plasticity 25 

The instrument corroborates our experience as to the effect of 
consistency in every respect. 

Consistency is not of very great importance. Changes due to 
consistency can practically never be of sufficient magnitude to 
render a nonplastic hydrate plastic or the reverse. For this 
reason consistency as judged by the eye will usually be close 
enough for practical purposes. 

In cases where greater accuracy is desired, as when comparing 
similar hydrates, the consistency must be considered and accu- 
rately measured. We have found it convenient to use the South- 
ard vicosimeter 22 for this purpose. This is a modification of the 
"slump test" which has been in general use on concrete for some 
time. The molded cylinder is 2 inches in diameter by 2% inches 
high. A sample of plaster gaged ready to put on a wall was tested 
and found to slump one-half inch; that is, when the cylinder of 
paste was removed from the mold, it deformed of its own weight 
until it was only 2 inches high. This was taken as the "normal 
consistency." In all plasticity experiments the material being 
examined is mixed with enough water so that it will show a slump 
of one-half inch when tested in the Southard viscosimeter. 

VII. EFFECT OF PROPORTION OF SAND UPON PLASTICITY 

The quantity of sand which is added to a lime paste to make a 
plaster is governed almost entirely by the plasticity. The mortar 
mixer adds as much sand as he can without making the mortar 
so lean that it can not be worked successfully. 

It is usually claimed, but never conceded, that a dolomitic 
hydrate will carry more sand than a high-calcium hydrate. Cer- 
tainly the dolomitic hydrate has the advantage of greater plas- 
ticity in the neat paste. As hydrate is replaced by successively 
larger amounts of sand the mixtures become leaner. Their plas- 
ticities approach each other and finally become equal when all 
of the hydrate has been replaced by sand. Since the plasticity of 
the sand is lower than that of the lime, it is evident that the 
replacement of lime by sand will cause a continuous decrease of 
plasticity. At a certain point it will be found that the plasticity 
of the mixture is so poor that it can no longer be used for the pur- 
pose intended. This point indicates the sand-carrying capacity 
of the lime. It is obvious that, in order to measure the sand- 
carrying capacities of different limes, it is essential that the sand 

84 Report of Committee C-n, Am. Soc. Test. Mat.; 1919. 



26 



Technologic Papers of the Bureau of Standards 



be uniform for all tests. A standard sand, to be used in testing 
plastering materials, is a necessity. It is worth noting that the 
process can be reversed, and, by the adoption of a standard lime, 
can be used to measure the lime-carrying capacity of different 
sands. 

Probably the sand-carrying capacity will be found to vary with 
the purpose for which the material is to be used. It was noted 
above that a lime must be in the 20-minute class or, better, to be 
used for finishing. Possibly a 1 5-minute mixture of lime and sand 



100- 



35 so- 



60- 



.? 50- 
?* /in. 




Fig. 8. — Effect of proportion of sand on plasticity 



will be satisfactory as a scratch coat on metal lath, while for brown 
coat we may be able to go still leaner, to the 10-minute class. 

Fig. 8 shows the plasticities of all proportions of lime and sand. 
It offers definite proof that the dolomitic hydrate does carry more 
sand than the high-calcium hydrate. It shows that a mortar 
made of dolomitic hydrate is more plastic than one made of high- 
calcium hydrate, regardless of the proportion of sand, up to that 
point where they are both so lean that there is little difference 
between them. It shows that the addition of a small amount of 
sand to a high-calcium hydrate slightly improves its plasticity, a 
fact which is frequently taken advantage of in practice. 



Measurement of Plasticity 27 

vni. CONCLUSION 

An instrument has been devised that will measure plasticity. 
We now have a tool with which to work on the important investi- 
gations into the cause of plasticity and the improvement of 
plasticity. 

At the instigation of J. J. Karley another instrument is now 
being built which is much simpler in design and on a much larger 
scale than the present machine. It is proposed to use this new 
plasticimeter to attack the problem of the plasticity of concrete. 

The instrument is available for use in writing standard specifi- 
cations for lime and gypsum. 

The investigation has been conducted during such a long period 
of time that it is difficult to make due acknowledgement to all 
whose thoughts and work have been included in the final product. 
Those who have contributed the most are, probably, J. J. Earley, 
W. E. Carson, E. E. Eakins, A. V. Bleininger, P. H. Bates, F. A. 
Kirkpatrick, W. B. Orange, S. K. Kaczorowski, and C. H. Bacon. 

Washington, January 15, 1920. 




LIBRARY OF CON °^^, 

029 942 429 







